DEVELOPMENT AND CHARACTERIZATION OF FeAl BASED COMPOSITES

Abstract

Iron aluminides are promising structural materials for industrial applications relative to typical steels and other engineering alloys due to its low density, low material costs, high melting point and excellent high-temperature oxidation resistance. Poor room temperature ductility and drop in the strength of iron aluminides above 550oC limits their role as structural materials. There has been considerable effort devoted to the improvement of mechanical properties by refining microstructure and by the introduction of a fine dispersion of second phase particles. Addition of carbon to the Fe–16wt.% Al alloy results in improvement in strength, machinability, creep resistance and resistance to environmental embrittlement. Carbon additions of 0.03 wt.% or more result in precipitation of Fe3AlC0.5 phase in these alloys, which imparts significant dispersion strengthening and leads to improvement in creep resistance. Though Fe3AlC0.5 (k-carbide) is a hard and brittle phase its presence in the alloys matrix does not affect the ductility of these alloys. This is because interstitial carbon competes with hydrogen for interstitial sites thus reducing the mobility of hydrogen atoms leading to brittle embrittlement. This susceptibility to embrittlement in moist environments is a major cause for poor room temperature ductility and machinability. Carbon additions to B2 FeAl based alloys having more than 20%Al results in the precipitation of soft graphite. The increase in the carbon content from 0.27%C to 1.1%C, results in the decrease in the poor creep rate, strength and stress rupture life. This may be attributed to the precipitation of soft graphite phase against the hard Fe3AlC0.5 carbide. The precipitation of graphite to limited extent also causes poor wear resistance of FeAl based alloy. A stronger carbide forming element has been proposed to avoid the graphite precipitation and improvement in the mechanical properties of FeAl based alloys. In the present research work attempts are made to develop FeAl based iron aluminides containing carbon. Transition metals like Ti, Zr are stronger carbide forming elements and avoid the graphite precipitation in FeAl based alloy. In the present work Ti/Zr additions resulted in the formation of alloy carbides which improved mechanical properties at room as well as elevated temperatures. The wear resistance of FeAl based alloys containing carbon have also improved due to Ti/Zr additions. ii The whole research work has been presented in eight chapters in the thesis. Chapter 1 represents the introduction about the intermetallics compounds and the difference in their mechanical and physical properties from the conventional alloys and metals. A brief introduction about the iron aluminides has also been reported in this section. Chapter 2 presents a critical review of the available literature on iron aluminides, their different types depending upon the content of aluminium. The chapter represents the effect of carbon additions on microstructure and mechanical properties of iron aluminides. Chapter 3 consists of formulation of problem, objectives of present work based on literature review and planning of experimental works. Chapter 4 consists of the experimental procedures employed for present work. Details of various instruments used to study the mechanical properties, thermal properties and wear bevaiour has been described. Various heat treatments given to specimens of various alloys are described in details in this study. Chapter 5 deals with characterization of microstructural and mechanical properties as well as the thermal properties of FeAl based alloys containing carbon on Ti addition. Chapter 6 describes the wear behaviour of FeAl based alloys containing carbon on Ti additions. The effect of applied loads and sliding speeds has been investigated and correlate with microstructures observed. Chapter 7 deals with the comparative studies of FeAl based alloys containing carbon on Ti/Zr additions in terms of microstructural and mechanical properties. Also the wear behaviour of these alloys has been studied. Chapter 8 describes the future directions in which these studies can be extended.